Interpretive Summary: The forests in the Northern Hemisphere are an important carbon sink. Rising ground-level ozone is reducing the potential for these forests to act as carbon sinks. In this study, we summarized all of the previous work to date on tree growth responses to elevated ozone concentration. Current background ground-level ozone (40 parts per billion) significant reduced the total biomass of trees by 7% compared to trees grown in charcoal-filtered control treatments. Elevation of ozone to 64 parts per billion, which is approximately the level anticipated for 2050 further reduced total biomass by 11%. The analysis also revealed that angiosperms (broad-leaf trees) are significantly more sensitive to ozone than gymnosperms (evergreens). This has important implications for the make-up of future forests. This study demonstrated that the carbon-sink strength of the Northern Hemisphere forests is already reduced by background ozone and will be even more reduced in the future if ozone continues to rise. Therefore, an important carbon sink that is offsetting global fossil fuel carbon dioxide emissions might be dimished or lost in the future. This paper provides critical information for policy-makers and managers of forest resources.

Technical Abstract:
The northern hemisphere temperate and boreal forests currently provide an important carbon sink; however, current tropospheric ozone concentrations ([O3]) and [O3] projected for later this century are toxic to trees and have the potential to reduce the carbon sink strength of these forests. This meta-analysis estimated the magnitude of the impacts of current [O3] and future [O3] on the biomass, growth, physiology and biochemistry of tress representative of northern hemisphere forests. Current ambient [O3] (40 ppb on average) significantly reduced the total biomass of trees by 7% compared to trees grown in charcoal-filtered (CF) controls, which approximate pre-industrial [O3]. Above- and below-ground productivity were equally affected by ambient [O3] in these studies. Elevated [O3] of 64 ppb, which is similar to levels anticipated for ~2050, reduced total biomass by 11% compared to trees grown at ambient [O3] while elevated [O3] of 97 ppb, similar to levels anticipated for ~2100, reduced total biomass of trees by 17% compared to CF controls. The root to shoot ratio was significantly reduced by elevated [O3] indicating greater sensitivity of root biomass to [O3]. At elevated [O3], trees had significant reductions in leaf area, Rubisco content and chlorophyll content which may underlie significant reductions in photosynthetic capacity. Trees also had lower transpiration rates, and were shorter in height and had reduced diameter when grown at elevated [O3]. Further, at elevated [O3], gymnosperms were significantly less sensitive than angiosperms. Taken together, these results demonstrate that the carbon-sink strength of northern hemisphere forests is likely reduced by current [O3] and will be further reduced in the future if O3 rises to elevated levels as projected. This implies that a key carbon sink currently offsetting a significant portion of global fossil fuel CO2 emissions could be diminished or lost in the future.